Electric motors are used extensively in manufacturing operations and it goes without saying that some means is required to control their operation, as well as to provide for protection of the motor and the environment in which the motor is located.
Consequently, it is common to include an overload protector in circuit with the motor which is operative to sense when an overload condition occurs and cut power to the motor. Even with the use of overload relays or the like, it is nonetheless required in many jurisdictions and, is good engineering practice, to include fuses in the motor circuit.
Where the motor draws relatively large amounts of current, it is also conventional to use a so-called "contactor" to connect the motor to power for starting purposes or to disconnect the motor from power to halt the motor's operation. Contactors are basically relays having electromagnetic switches in series with the motor and a source of power. Conventionally, a lower voltage circuit is utilized to provide power to the electromagnet controlling the electrical contacts and thus, a transformer will conventionally be provided to reduce the voltage of the line to that suitable for purposes of control of the contactor. Not infrequently, the various control circuits are fused as well.
The control circuit may take on a variety of forms. Push button or other types of switches may be utilized individually for start, stop, and jog functions for the electric motor being controlled. Various interlocks may be provided to prevent operation of the motor unless several things are occurring simultaneously or certain conditions are not present. Pilot lights may be employed to provide an operator with an indication of the status of the motor and its control circuits. The list goes on forever.
Conventionally, where several motors are involved, a so called motor control center may be employed. A motor control center is typically a panel that includes a plurality of bus bars which are connected to the power line or lines. The panel is constructed so as to be capable of receiving a plurality of motor control units, with a motor control unit for each motor to be controlled.
A typical motor control unit may include a fuse holder for fuses that are placed directly in line with the electric motor, a contactor for stopping and starting the motor, an overload relay for stopping the motor in the event an overload is sensed, a transformer for providing a low voltage for control circuits, a terminal block through which control and power circuits may be connected to the contactor and motor, fuses for the low voltage circuit driven by the transformer, and a disconnect switch which may be opened or closed to connect or disconnect all of the above from the power supplied to the motor control unit by the bus bars.
Packaging all of these components in a small space has been difficult. For one, the components must be sufficiently spaced from one another as well as from the housing constituting the base of the motor control unit so that there will be no arcing to ground should short circuits occur. At the same time, the constructions must be such as to withstand any pressure that might build up during a malfunction if metal, heated by electrical current, vaporizes.
Access to the various components must be relatively easy. For example, fuses in the main motor circuit will blow with some frequency, particularly when the motor is being started, because of the heavy current draw at such times.
It is also necessary to achieve access to all of the various terminals in the unit so that they may be easily initially wired and periodically tightened.
Heretofore, many commercially available types of motor control units have required virtual complete removal from the motor control center for the purpose of changing fuses and/or tightening connections. Where such is not required, frequently the unit is quite large so that easy access can be had to all components within the unit for these purposes. In the case of the former, the added effort that is required to provide maintenance and/or change fuses is costly, and therefore, undesirable. In the case of the latter, more space than is desirable is required to house the motor control center because of the large size of the individual motor control units.
The present invention is directed to overcoming one or more of the above problems.